If you live on or near the East coast of the US and you’ve been wondering why it’s been so cloudy lately, here’s the reason:

That’s a low-pressure system that’s been squatting over the Great Lakes region for a few days, as seen by NASA’s Aqua Earth-observing satellite [click to centrifugenate]. It stretches clear across the country north-to-south; you can see the Gulf of Mexico at the bottom of the picture.

Why is it comma-shaped? Because the Earth rotates. Seriously.

The Earth spins once per day, and is about 40,000 km (24,000 miles) around at the Equator. That means someone standing there makes a circle that big once per day, moving at a velocity of about 1700 kph (1000 mph). But someone standing at the pole isn’t making a circle at all; they would just spin in place once a day. At an intermediate point, say a latitude of 45°, someone would be moving around in a circle at a velocity of about 1200 kph (700 mph).

Now imagine you’re standing on the Equator, moving east at 1700 kph. I suddenly magically transport you to 45° north latitude. What happens? Well, you’re still moving east at 1700 kph, but the ground is only moving east at 1200 kph. That means relative to the ground you’re moving faster to the east by 500 kph! To someone standing there, you’d pop out of thin air screaming eastward nearly as fast as an airplane. Better pack a parachute.

The opposite is true if you’re at the north pole, not moving at all, and I whisk you south to 45°: now the ground is moving east of you at 1200 kph. To someone already there, you’d appear moving west at that speed. It’s all relative.

Weird, isn’t it? But it’s a natural consequence of living on a rotating ball.

Now imagine you have a low pressure system over the Great Lakes (or simply look at the picture above). Air is rushing in, toward the center, from all directions, including north and south. But those masses of air are moving east at different velocities due to the rotating Earth’s speed. Air from the south is moving east more quickly than the center of the weather system, so (in the picture) it moves to the east (right) as it heads north. Air moving south is moving slower than the ground, so it bears west (left). This sets up a counter-clockwise rotation, just like you see in the picture.

That’s why hurricanes rotate counter-clockwise in the northern hemisphere. In the southern hemisphere, this is reversed: air moving south is traveling faster than the ground beneath it, and slews east; air moving north slews west, and you get clockwise rotation.

This almost seems like a force, doesn’t it, something pushing the air around? In many ways it does act like a force, though it depends on whether you’re looking at it from the ground, rotating with the Earth, or from space, watching the Earth spin beneath you. This whole thing was first figured out in detail by Gaspard Gustave de Coriolis in the 1800s, and we name it after him: the Coriolis effect (or, sometimes, the Coriolis force).

This only works on large scales, since the effect is caused by the change in rotation speed with latitude on the Earth. It has nothing to do with your toilet; the water would flush the same way if you’re in Australia or Austria (the water going into the toilet boil spins due to the way the flow is directed by the bowl itself, and not by the rotation of our planet).

But on large scale it drives megatons of air one way or another. So if you’re in the eastern US and feeling glum due to gray weather, remember: there are vast forces at work making it that way, and there’s not much you can do about it. And if that feels like spin, well, that’s because it is.

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The Coriolis effect is also important in ballistics too. I’m fairly certain the Germans had to take it into account when using the Paris Gun in WWI. When you shoot a projectile 75, 100 miles, it can matter!

If rail guns are ever successful, I imagine the Coriolis effect will have to be accounted for as well.

Nice explanation of the Coriolis Effect. Thank you, too, for debunking the toilet/sink effect. It’s one of the most durable myths around, along with the Full Moon Effect and the idea that your hair and nails keep growing after you die.

I don’t seem to remember them much 20-30 years ago. And it seems they’re forming much more often.

Is it simply because of better satellite tech and meteorological equipment? Me just not paying attention?

I’m amazed how they can stretch as far as a hurricane would, but tend to be gentle giants. They simply don’t have the same energy. That is until they hit cold arctic air over New England and dump us in a few feet of snow.

What a wonderful description. After years of reading about this effect, I finally understand it well enough to explain it to someone else. You’ve got Isaac Asimov’s gift with explaining science, Phil. Thanks.

I remember learning about this sort of thing as a kid from Larry Niven short stories that dealt with the physical and social complexities of his “transfer booths”, which actually can magically transport you 45 degrees north latitude. They also got into potential-energy issues when changing altitude.

@Francesca: Even then, only if it is perfectly smooth and has a day or so to settle down before flushing. If you squirted a little water there with just enough speed to go around the bowl in an hour, that’s 24 times as fast as the Earth’s rotation. The Wikipedia article on the Coriolis effect has a little more detail on this.

13 (Ryan) – Theres nothing unusual about this. Its a big cutoff low pressure area. The energy from the trough (low pressure) got separated from the jetstream so there is nothing to steer it away so it just sits in place until the next storm comes in an move it.

[9.] Andrés,
That’s correct! On an average, the distance to the centre of the Earth is longer the closer you are to the equator, so the gravitational force on the object is smaller. That’s one reason why you find rocket launch sites situated at relatively low latitudes — less gravitational force to “struggle” against.

By the way, the centripetal force is more a classification of force, than a unique one like gravity. Any other force in various situations can be classified as centripetal. Consider these examples: the tension in a string, when you whirl it around with a stone tied at the other end; the electromagnetic force due a proton on an orbiting electron, in the classical picture of the atom, and of course, the force of gravity on you, me, a satellite or the Moon.

I never really understood the ‘water goes down the toilet in the opposite direction in the southern hemisphere thing’ until I actually went to America.

The toilets are different.

In America every toilet seems to be set up with a built in injection of water that comes in from the side, fills the bowl, and then suddenly – whoosh, everything gets sucked down. Often you can’t even see the cistern – I assume it is usually in the wall behind the toilet itself.

Standard toilets in Australia and New Zealand don’t do that. The bowl isn’t filled unless the S-bend gets blocked, which is bad. Instead, water is flushed from the (always present) cistern into the bowl in very rapid, large volumes. It’s almost exactly like pouring a couple of buckets into the bowl from multiple sides at once.

Water isn’t swirled into the bowl from the side at all. There’s no whoosh. It’s all just dumped in, practically and without ceremony.

So not only does the water going down the toilet in the southern hemisphere not go the other way… The deception is deeper than this.

There is no spinning of the water going down the toilet at all!

Of course, I can’t comment on countries in the Southern Hemisphere that are not Australia or New Zealand – but still. I hope your minds are all appropriately blown.

I’ve always wondered why NASA never ran an experiment to test the implementation of rotational “gravity” (and how humans cope with it). Seeing as how it could potentially solve most of the health issues associated with space travel, and might well be necessary for missions to Mars… No massive Clarke cylinders necessary, either – all you’d really need would be a small habitat with a decent RTS and a counterweight (also with an RTS) with a winch on it, and you could test a variety of radii and RPMs. Not that it’d be easy, or anything, but it’d be relatively trivial compared to the engineering and investment that went into the ISS, for instance.
Still, I can imagine that that’d be an unpopular assignment for astronauts. Nausea aside, imagine getting to space and not getting to fool around in microgravity when you get there

Also, thanks to Phil for the clear explanation. I had a hard time wrapping my head around the Coriolis effect back when I was in school. Most books simply don’t explain it in a way that you can visualize.

#10 Steve R: To someone standing there, you’d pop out of thin air screaming eastward nearly as fast as an airplane.”……..wonder how Star Trek transporters took that into affect?
That’d make a helluva weapon. You’re in orbit, and want someone on the surface to not exist anymore? Just teleport a bowling ball a few feet to the west of ‘em. Mach 25 strike. Literally Hell, at that speed a child’s party balloon could probably kill you.

#22@Steve: Eh, you could argue that the parenthetical assists are helping us to develop a sense for translating between the two systems. As long as Imperial units are used for practically everything in the US, it’s good to have a knack for that. (And of course, the US is probably never going to switch to metric until some critical mass of the population has that.)

Off-topic, but a couple of weeks ago there was a post about an article Phil contributed to about starting off in astronomy for beginners, including what sort of telescopes to look for. Now I’m seeing a banner ad that says “Telescopes for beginners”.
Google, you done it again. Not sure whether to be impressed or creeped out (Ok, I don’t know if Google handles ad placement for Discover, but I wouldn’t be surprised)

What happens if someone is magically transported from the equator to one of the poles? Would they be launched into space at a 90 degree angle to the axis of the earth at a speed of 1700 kph? If so, how far into space would they go before gravity brought them home?

The weird thing is that since this system has been skirting Toronto for last few days (yes, that big Toronto on the North West corner of Lake Ontario), so we’ve had mostly warm sunny weather here. Sucks for everyone else though.

Navneeth, not really. You get a minor increase in altitude from a polar location, but that can be offset by a mountain launch site. Usually, though, launch sites are coastal. The real reason for equitorial launch sites is the speed advantage from using Earth’s rotation. Saves energy for launch.

Steve (treelobsters), relative velocities will be substantially lower, so the centrifugal force reduction will be less and the relative velocities will be lower. But there will still be some variance from the rotation.

27. Daniel Schealler: Those are what we call ‘power flush’ toilets. I assume by ‘cistern’, you mean where the water is stored before flushing? We just call it the ‘tank’. Anyway, power flush toilets don’t have a cistern at all; they use the water pressure directly, instead of accumulating it with gravitational potential energy like a cistern. They’re most efficient when you have a number of toilets in one place, because you need a larger pipe to provide that much water, and it’s not so much worth it for just one toilet. So you tend to see them more in business places and public restrooms. You almost never find them in single residences, and rarely even in apartment buildings. There, you usually see toilets with cisterns.

@34 Steve: What would happen with a planet that was tide-locked? I’m thinking of an Earth-like planet in a close orbit around a relatively cool star. Technically it’s still rotating but only once per orbit.

That’d be really interesting. I’m not sure how long such a planet’s year would be. I’m guessing we could assume that if there were a Coriolis effect, it’d be much, much weaker then on Earth.
I’d think in that case the biggest factor in shaping weather patterns would be heat exchange between the “hot pole” and the “cold pole”. You’d definitely have some sort of very strong winds generated by that heat gradient, but of what sort, I don’t know. Maybe something like jet streams, but I’m betting that they’d be much more chaotic then on Earth – jet streams on Earth are shaped by Hadley cells which are constrained by the Coriolis effect.
I can’t begin to imagine what sort of weather a planet like that would have, but I’ll bet it’d keep a lot of meteorologists busy

@34 Steve: One great vision of a tidally locked planet is Larry Niven’s story ‘Flare Time’. You can read it here if you don’t mind (probably) bending the law a bit:http://www.docin.com/p-225971051.html

Understanding the why of those damnable gray clouds hanging around overhead doesn’t make me curse them any less. We’re in the dark o’ the moon phase, and I want to photograph my stars and milky way!! It’s not Coriolis, it’s Murphy’s Law.

Now let’s look at the REAL reason that front is pushed swirling back to the west. Ottawa and Washington D.C. are positioned to the east side of the continent. All that hot air is blocking the system from moving off.

That’s why hurricanes rotate counter-clockwise in the northern hemisphere. In the southern hemisphere, this is reversed: air moving south is traveling faster than the ground beneath it, and slews east; air moving north slews west, and you get clockwise rotation.

I wonder what happens when hurricanes/typhoons cross the equator, as they almost certainly do regularly in the Pacific??

I wonder what happens when hurricanes/typhoons cross the equator, as they almost certainly do regularly in the Pacific??

That’s a good question, so I Googled around a bit. If what I’ve found is true, there are no recorded cases of hurricanes/typhoons crossing the equator (I was surprised, too). They’ve come near it, but never crossed. There’s some speculation on whether it’s possible and if so how long it’d take for the Coriolis effect to stop the storm’s rotation – there’s also disagreement on whether the storm would simply die there, or start spinning the other direction. It seems to be completely hypothetical, though.
If what I’m reading is correct, the lack of Coriolis effect at the equator keeps cyclonic storms from forming – a low pressure area is simply “filled in” without inducing a pattern of rotation that preserves the low pressure.
Generally speaking, storms appear to move away from the equator, following the prevailing winds aloft. Check out “Hadley cell” on Wikipedia for more on this.

Phil, please *don’t* drop the Imperial Units in parentheses. Although I was a science major in college, and have been science-oriented all my life (if not always directly involved with higher science in my career), I still think in terms of English units and probably will until until my end of days. I always have to translate in my head from one to the other.

I’m not a Luddite. Far from it. It’s just that that’s the way my brain is wired and by this time it ain’t gonna change that much. Can’t help it; it’s just the way nearly all people in the states are who do other things for a living besides think about science all day, notwithstanding certain exceptions to that rule who’ve posted on this thread.

You probably already understand this (or you wouldn’t be doing the parentheses thing), but if you want the casual layman to be able to quickly grasp various distances and other measurements, English units in parentheses are indispensable.

@ 42 & 43 The natural tendency for hurricanes/cyclones is to head for the nearest pole but they’re influenced by steering winds in the upper atmosphere and can be blocked, and thus move sideways, by high pressure systems. They also generate away from the equator and thus already “know” which pole to head for. That’s my very simplified version.

I would like to take advantage of “Coriolis effect” being on the front page –I thank you our host for that since is one of my favourite scientific doubts that is never properly answered.

My doubt is that in the way Coriolis is normally explained, it just an appearance. My point is that if we have an object in the equator and we make its latitude grow, the radius of the circle over which it revolves -around earth axis- gets smaller and this alone would make it go faster, as the proverbial skater spins faster when folding the arms. So it is not that “people see it going faster” but it going faster, since angular speed is absolute.

The explanations where Coriolis is presented as an “appearance” crash in my mind with the skater spinning because she is really accelerating (and no external force have been applied to her).

When we say –as Phil did- that we push the object towards a higher latitude “without doing any force” I feel uneasy because if you do so without doing any force, the object will accelerate. In fact, in some contexts the Coriolis force is defined as the force you need to do when while reducing the revolving radius of an object, you stop it to accelerate (“Phil case”).

To summarize my point, Coriolis is presented normally as something similar to an optical illusion that appears in “non inertial systems” but for me is a manifestation of the conservation of the angular momentum at the same level of reality than the proverbial spinning skater that folds her arms.

because if Coriolis is an illusion, the acceleration of the skater should also be an illusion, should’nt it?

Well, yes, but the difference is less than 1%, whereas the main advantage of an equatorial launch site is that your rocket is already moving at 1700 km/h relative to the centre of the Earth even before it leaves the launch pad.

As long as Imperial units are used for practically everything in the US. . .

Except that the US only uses some imperial units, mixed in with some that are all its own.

For example, IIUC, you don’t use the yard very often (quoting distances less than a mile in hundreds or thousands of feet instead); the stone (14 lbs) and the hundredweight (8 stone) get no look-in at all; and the Imperial ton (2240 lbs, or 20 cwt) is called the “long ton”, whereas the term “ton” is often used to mean the “short ton” (2000 lbs); and the US gallon (3.8 L) is very much smaller than the Imperial gallon (4.5 L).

The weird thing is that since this system has been skirting Toronto for last few days (yes, that big Toronto on the North West corner of Lake Ontario), so we’ve had mostly warm sunny weather here. Sucks for everyone else though

Interestingly, I cycled past the village of Toronto at the weekend. Bit it’s not in Canada. It’s in County Durham, near Bishop Auckland.

@Liath You can get a quick estimate of the height from conservation of energy, potential+kinetic =constant. So the transported being would drop to zero velocity at mgh = (mgR + mv^2/2), or R+v^2/(2g) [R=the Earth’s radius] They’d reach a height of v^2/(2g). with v=1700kph and g~12740kphph I get approximately 11.3 km into the air.
This would be slightly reduced by air resistance (the reduction from the non-spherical shape of the Earth is less than .05km). It might be an interesting exercise to see how hot something (or someone) would get from heating due to friction.
Wait, g is one earth diameter per hour squared? Why did I not know this?

A comma is a nice way to describe it. I live under that thing, which looks more like a rain turd coiling itself right on top of my home state, delivering bleak, miserable rain for nearly five days now.

@49 Nigel Depledge: Well, yes, but the difference is less than 1%, whereas the main advantage of an equatorial launch site is that your rocket is already moving at 1700 km/h relative to the centre of the Earth even before it leaves the launch pad.

I always thought that one main concern was a greater choice of orbital inclinations. A launch site on the equator, for instance, can launch directly into an orbit of any inclination. A launch site at, say, 46 degrees latitude, on the other hand, can’t launch directly into an orbit that’s inclined by less then 46 degrees to the equator. You can change your orbit once you’re up there, of course, but it takes fuel.
Incidentally, the Baikonur Cosmodrome is at 46 degrees latitude, which is the stated reason for the ISS’s orbital inclination of 51 degrees. Cape Canaveral is at about 28.5 degrees, but a greater orbital inclination is necessary so that the Soyuz missions can be flown.

@ Joseph G (56) –
Good point. Maybe that reason is more compelling than the extra velocity you get, but I stil think that the extra velocity is more significant than the difference in gravitational force.

#57 The main reason to operate landings and take-offs in spinning objects as close to the equator as possible is that less energy per useful kg of load is needed. This implies more useful load for the same rocket or less rocket for the same load. Baikonur is a very bad place and the reason why russian rockets are more powerful than french ones. Guyana is so closed to the equator that a firework rocket can go to the orbit. Also you can see how moon landings tend to be in the equator.

PS : I misread Phil about Coriolis. He was saying the same that I always say but shorter and more to the point. In fact he said something that many people does not accept that is “the angular speed of the object increases to maintain its linear speed”.

Everything on and in a plane moves or does not move as one entity. If you are moving faster or slower than I am. The distance between us must change. Since the distance between us does not change your claim of different speeds has zero evidence.